A high recording density in an optical recording medium, for example, an optical disc is usually realized by a method of making wavelengths of irradiating light shorter and making numerical aperture of an objective lens larger. However, by further combining this method with forming a multilayer of information-retaining layers, that is, information-recording layers, it will be possible to realize a remarkable high-density recording.
In the optical recording medium, however, if a multilayer of information-recording layers is formed, or a plurality of information-recording layers are laminated, a problem is posed that, as the information-recording layer becomes distant from a principal plane (incident plane) on the side where light enters the optical recording medium, a less amount of light will reach the layer, because each information-recording layer absorbs light.
This is an unavoidable problem when a single light source is employed for the plurality of information-recording layers.
Moreover, the same applies to return light from the information-recording layer, namely, detected light in reproduction. The return light from the information-recording layer at a distant position from the incident plane is made to return by permeating other information-recording layers positioned on the incident-plane side of optical recording medium while the light is detected, so that the amount of that light decreases.
Therefore, the more distant the information-recording layer becomes from the incident plane, the higher reflectance needs to be set. At the same time, the modulation factor needs to be high to the same extent.
Incidentally, a detection principle of the optical recording medium such as an optical disc can generally be divided into two ways.
One of them is a phase-modulation system typically employed in CD-ROM (Compact Disc-Read Only Memory) disc, what is called CD-R or DVD-R, that is, write-once CD or DVD (Digital Versatile Disc), and the like.
In this phase-modulation system, while reflectance within an information-recorded mark is made equal to reflectance in its circumference for making the reflectance uniform, an optical-path length in the information-recorded mark is made different from an optical-path length in its circumference for making the phase of reflected light change to cause an interference in a light spot, with the result that the amount of return light is made to change.
The other of them is a reflectance-modulation system typically employed in optical discs using a phase-change material such as what is called CD-RW, DVD-RW, or similar which is a rewritable type CD or DVD. In this reflectance-modulation system, by utilizing that reflectance within the information-recorded mark is lower than reflectance in the circumference, a change in the amount of return light due to the difference of reflectance is detected directly.
Incidentally, the above-described information-recording layer in the phase-modulation system typified by that of CD-R or DVD-R is conventionally formed of a recording film composed of an organic dye. An optical recording medium whose recording film is thus composed of the organic dye material has such advantages as it is easy to manufacture, simple to handle the material, and low in cost.
In the phase-modulation system, it is required that the reflectance should be unchanged before and after recording to be maintained almost uniformly, in order to cause an effective interference with the information-recorded mark and its circumference.
To this end, practically the CD-R or the like is arranged such that the reflectance may be unchanged before and after recording by making the information-recording layer formed of the recording film of organic dye into a multilayer structure in which a thick film of metal such as Au or Ag is laminated.
However, when the information-recording layer is made into a multilayer structure in which the thick metal film is laminated, the problem arises that incident light may not reach the information-recording layer located at a distant position from the incident plane.
Accordingly, it is impossible to apply, without any change, such a conventional design for the recording film composed of organic dye to the information-recording layer having the multilayer structure.